docs: def add_link(self, obj, tag=None):
          """
          add_link(obj, tag=None)
  
          Add a link to a RiakObject.
  
          :param obj: Either a RiakObject or 3 item link tuple consisting
              of (bucket, key, tag).
          :type obj: mixed
          :param tag: Optional link tag. Defaults to bucket name. It is ignored
              if ``obj`` is a 3 item link tuple.
          :type tag: string
          :rtype: :class:`RiakObject <riak.riak_object.RiakObject>`
          """
          if isinstance(obj, tuple):
              newlink = obj
          else:
              newlink = (obj.bucket.name, obj.key, tag)
  
          self.links.append(newlink)
          return self._robject